Optical waveguides are commonly utilized in optical communication applications. Optical waveguides usually include a core layer sandwiched between a lower cladding layer and an upper cladding layer. Typically, the core layer has a higher refractive index than the lower and upper cladding layers.
Traditionally, silica was used to form the core and the cladding layers, but the core layer was doped (for example, with germanium, phosphorous, or titanium) in order to increase its refractive index relative to the cladding layers. In recent years, there has been increasing focus on fabricating optical waveguides having greater index contrast between the core layer and the cladding layers by using plasma enhanced chemical vapor deposition (PECVD)-grown silicon nitride or silicon oxynitride core layers.
However, high optical losses (for example, 5-10 dB/cm) are often observed in PECVD-grown silicon nitride and silicon oxynitride core layers at infrared (IR) wavelengths used in optical communication applications. It is believed that the losses originate from Si—H, N—H and —OH bonds that are incorporated into the core layer during PECVD from silane, nitrous oxide, and/or ammonia gaseous precursors. It is therefore necessary to anneal the core layer at high temperatures in order to remove the Si—H, N—H and —OH bonds in the core layer.
Silicon nitride-based and silicon oxynitride-based ridge waveguides are thus typically fabricated using the sequence illustrated in FIGS. 1(a)-1(e). First, as shown in FIG. 1(a), a lower cladding layer 10 (for example, a silicon dioxide lower cladding layer) is deposited on a silicon wafer substrate 12. Then, as shown in FIG. 1(b), the silicon nitride or silicon oxynitride core layer 14 is deposited on the lower cladding layer 10 using PECVD. Next, as depicted in FIG. 1(c), the structure is annealed 18 and then, as shown in FIG. 1 (d), the core layer 14 is patterned. Finally, as shown in FIG. 1(e), an upper cladding layer 16 (for example, a silicon dioxide upper cladding layer) is deposited on the annealed patterned core layer 14.
Silicon oxynitride core layers having a refractive index above about 1.6, however, can develop cracks as a result of high temperature annealing (for example, annealing at about 1000° C. or above). The cracking is due to the differing thermal expansion coefficients of the silicon oxynitride core layer, the cladding layers, and the silicon wafer. The differing thermal expansion coefficients lead to unacceptably high values of induced tensile stress within the layers. The cracking problem can be even worse with silicon nitride because silicon nitride films have higher tensile stress than silicon oxynitride films.